296 METALLURGY OF IRON AND STEEL. sorbed at a critical point is due to the change from one structure to another, or whether both the change and the heat are due to some unknown molecular phenomena. The next section will discuss the structures and forms which are best known and which must be studied to understand the effect of heat treatment. SEC. XVj.—Definitions of the different structures seen under the microscope.—The microscopic examination of almost any piece of steel properly polished and etched will show that it is not entirely homogeneous, but that it is usually made up of at least two different forms of matter. It will not do to say that it is always made up of different substances, for it is generally agreed that some of these forms are allotropic,* the particular forms present in any one piece depending upon the way in which that piece has been heated and cooled. Considering all variations in heat treatment, the following forms will be encountered by the investigator: aus-tenite, martensite, pearlite, cementite, ferrite, troostite and sorbite. Austenite is produced only by quenching steel containing more than 1.30 per cent, of carbon in ice water from above 1050° C. Its appearance is intended to be represented by the white portion of No. 1, Fig. XV-B, but this may be cementite in spite of the fact that the piece was steel containing 1.40 per cent, carbon, one-quarter of an inch thick, and was quenched in melting ice from a dazzling heat. Even under these conditions it is impossible to obtain a large quantity of austenite, since the tendency to revert to the next' form is very strong when the proper temperature is reached. The theory of austenite, as well as of martensite, will be taken up in Section XYo. At about 1050° C. a change occurs, and in this grade of steel quenched below th'\s point and above A^ the second form, martensite, appears. This phase, together with a certain amount of cementite or of ferrite, depending on the carbon content, is found in carbon steels containing less than 1.30 per cent, of carbon quenched at &ny point above Ar13 as will be shown in Table XV-M. Martensite is the constituent which confers hardness on steel and corresponds to the maximum hardness obtainable by • The word "allotropic" Is used by some of the metallographists to designate the character of the metallic aggregates. This Is not strictly correct, since allotropy refers to unlike forms of the same element, while the different metallic aggregates found In microscopical inyestigations of masses of steel are not elements and are not of the same composition. The term "phase" was introduced by Gibb and is used later in this discussion.